1. Field of the Invention
The invention generally relates to an apparatus for transferring a fast running dried fibrous web from a first location to a second location along a predetermined pathway. More specifically, the invention relates to an apparatus for transferring fibrous sheets or webs, such as those made from paper, from a first location to a second location at improved levels of invariability and efficiency by minimizing, undesirable motion thereof.
2. Description of the Prior Art
The production of webs or sheets of material from a plurality of fibers at high rates of efficiency and uniformity can be very difficult. For example, in the production of paper products such as tissue, all phases of the production cycle, and in particular those at the dry end of the operation, must be performed at high levels of quality and invariability. As a result, each aspect of the process must be rigidly controlled to every extent possible.
Relatively fragile types of paper such as tissue present particular production problems for a variety of reasons. In a typical tissue making operation, the fibrous web is formed, then dried using a dryer such as a Yankee dryer (such terminology being familiar to those of ordinary skill in the art). The web is then removed from the dryer, such as by scraping it off of the dryer using a device such as a doctor blade, and transferred to the next processing stages (e.g generally calendering,, scanning and reel up). This point in the production cycle is therefore known as the "dry end", since the process steps at this point of the operation are performed on a dry web.
After removal from the dryer, the fibrous web is typically transported through a calender, where the web is pressed between a pair of rollers, and a scanner, where the web can be scanned for the presence of obvious defects as well as physical properties such as basis weight, moisture, and caliper. A slitter for slitting, the web longitudinally, usually into two halves of substantially the same width, may be provided downstream of the scanner. The web is then wound up on a reel for transport to a next operation such as cutting, of the web into smaller sections, etc.
It is at this dry end that a number of production inefficiencies are experienced, for a variety of reasons. For one, the dry end of the production cycle is a discontinuous process, as the doctor blade becomes worn and must be changed periodically. Similarly, defects formed in the paper, production errors, and the like can require that the operation be halted and begun again. Despite the discontinuous nature of the operation, in order to produce a high quality end product, it is important that the product produced remains uniform in its physical properties such as caliper, softness and stretch, as well as thickness and invariability.
The primary factors affecting dry end machine efficiency are lost time with no paper on the reel and the amount of paper rejected at paper breaks. On most high speed machines, the paper web roll is kicked out at a paper break because it is difficult to make a turn-up on a half or partial-size roll, and if the roll is small, it is generally rejected. Other items affecting the efficiency of the machine at the dry end are down time during, doctor blade changes, paper breaks, tail threading failures, turn-up failure, downtime for dry end cleaning (which is required to avoid web breaks caused by dust and dirt falling, down onto the web during processing), lost process control, and roll top and bottom waste. Roll bottom waste is caused by the radially inner paper web adjacent the reel spool having to be rejected, and roll top waste relates to the radially outer paper lost at the top of the web due to the kick out phase, and/or by taking samples for testing, and/or roll handling after the tissue machine.
In an efficiently operated machine, the threading of a new tail after doctor blade changes and web breaks does not take more than a few seconds. If the threader is out of adjustment, several minutes can be lost during each attempt to thread the new tail. In addition, paper may plug, the tail chutes, requiring time-consuming cleaning, of the whole dry end. Therefore, it is desirable to minimize the number of times the machine requires threading (for example, by minimizing the number of web breaks).
During, fast processing at the dry end, the webs or sheets of material have a tendency to experience undesirable motion which is commonly referred to as web flutter. As should be apparent to those of skill in the art, any motion which is uncontrolled has the ability to affect the quality and uniformity of the end product. Therefore. it is generally desirable to minimize such web flutter to the extent possible.
Prior web stabilizers or flutter suppressers are described in U.S. Pat. Nos. 4,321,107 to Page and 3,650,043 to Overly et al. However, heretofore, such devices have not been able to provide optimal amounts of flutter suppression while maintaining optimal processing speeds and conditions.
Another method for stabilizing a running web at the dry end is described in U.S. Pat. No. 5,738,760 to Svanqvist et al, the disclosure of which is incorporated herein by reference. The '760 patent describes a method of improving web transfer by providing a substantially web-wide support surface having, an upstream end and a shape conforming to at least a portion of that of the predetermined run of the web, with the support surface being located in a position adjacent to that of the predetermined run and extending from a first device to a subsequent device. The device described in the '760 patent is adapted to create a flow of air in the direction of the web run along the support surface by supplying pressurized air of a first pressure, e.g., from a fan, through a pipe member across the support surface in a cross-machine direction adjacent the upstream end of the support surface. A flow of air is also provided downstream of the first flow along, at least one further line across the support surface in a cross-machine direction. The flow of air forms between the web and the support surface an air layer of reduced static pressure, which serves to stabilize the web against flutter.
The apparatus described in the '760 patent uses a series of consecutive substantially web-wide plate members which define a substantially web-wide support surface having an upstream end and a shape conforming to at least a portion of that of the predetermined run between the two devices. The support surface is located in a position adjacent to that of the predetermined run and extends substantially all the way from the drying section to the reel up. Each of the plate members defining the substantially web-wide support surface has a leading edge and a trailing edge, and the trailing edge of at least one of the plate members is located spaced from and upstream of a leading, edge of an adjacent one of the plate members so as to form a plate assembly having a first slot-shaped gap between the plate members for the passage of air therethrough. The '760 patent describes that the apparatus includes a plurality of plate member assemblies, with each assembly having, an upstream edge and a downstream edge. The downstream edge of one assembly is located spaced from the upstream edge of an adjacent assembly so as to form a second slot-shaped gap between the assemblies for the passage of air therethrough. The patent describes that by permitting air to pass through the second slot-shaped gap, it is possible to maintain optimum web transfer conditions by supplying additional air through the pipe member of the subsequent plate member assembly. In order to remove dust from the system, the patent describes that the dust is removed through the second slot shaped gap by way of an air flow, the size of which is maximized by locating the downstream edge of one plate member assembly at a slightly larger distance from the predetermined run of the web than the distance of the upstream edge of the adjacent plate member assembly to the predetermined run of the web.
While the apparatus described in the '760 patent has enhanced the dry end processing of such fibrous materials, optimal processing results have yet to be achieved. In particular, the inventors of the apparatus which is the subject of the instant application have discovered that the vent gaps between the multiple plate members of the '760 apparatus tend to vacuum dust from the sheet and deliver it to the area above the plate members. The dust can build up and ultimately interfere with the operation of the machine. The amount of dust accumulated around the dry end of the machine varies dramatically depending on the type of fibers used, the basis weight of the web, the machine type, etc. Therefore, to ensure safe and efficient machine operation, the machine must be monitored to prevent dust build-ups, and dust must be cleaned away frequently.
In addition, the inventors of the instant invention have found that the air supply means (i.e., the pipe member and gap) of the device described in the '760 patent tended to become plugged with fiber, which in turn can interfere with the flow of air therethrough and which can allow the moving, sheet or web to fall away from the plate members. As will be appreciated by those of skill in the art, an improper orientation or positioning of the sheet or web relative to the processing apparatus can lead to the formation of defects in the end product or to interruptions in the processing operation as a result of sheet or web breakages. Therefore, the machine must be monitored for dust and fiber accumulation and cleaned frequently to prevent the build up of such materials and undesirable clogging of the air supply nozzles.
An active web stabilizer is disclosed in published PCT Appl. No. WO 99/29603 to Stenz, et al. An air nozzle is provided at the leading edge of the stabilizer and directs air flow first around a curved surface and then alone, a subsequent working surface. As illustrated in FIG. 6, multiple units of the active stabilizers can be closely spaced without touching in the machine direction such that spent air can leave through an exhaust passage between each of the adjoining units. As noted above, however, such an arrangement of multiple spaced units can cause dust to be exhausted through the gaps between adjoining units, thus creating the possibility of undesirable dust build-up.
Accordingly, a need exists for an apparatus which can minimize web flutter at the dry end of a fibrous web manufacturing operation, while enabling the web to be processed at desirable rates of efficiency and minimizing dust build-up.